This invention relates to techniques for enhancing oil recovery from ageing fields or low-pressure reservoirs. In particular the invention offers developments in gasification processes adapted to assist in driving oil from subterranean formations, or in converting said oil to useful gaseous products.
The process of gasification of subterranean carbonaceous fossil residues such as coal, lignite, oil shale, tar sands, and heavy oils in fields where recovery is difficult due to insufficient pressure to drive the oil to the surface, has been described in the literature and some processes have been operated commercially.
An in situ gasification process to be applied to an underground formation of carbonaceous material is described in U.S. Pat. No. 4,382,469. In the proposed process, a controlled direct current is passed through the formation. That reference also mentions several other prior art gasification methods which are described in U.S. Pat. Nos. 849,524, 2,818,118, 2,795,279, 3,106,244, 3,428,125. These methods generally have the same objective i.e. volatilisation or pyrolysis of the carbonaceous material to drive off gaseous hydrocarbon products, i.e. fuel gas.
A further in situ gasification of subterranean carbonaceous deposits is described in U.S. Pat. No. 4,461,349, wherein a pattern of bore holes is formed to provide in parallel a row of gas injection wells and a row of production wells. Oxygen containing gas is injected into the subterranean coal field to enable a combustion front driving a resultant gasification of the coal to be formed. The front drives the gas formed by thermal conversion of the carbonaceous deposit towards the production wells where thermocouples or the like detectors may be relied on to trigger a shut-down procedure to prevent combustion at or in the production wells. The process described there is said to be particularly suited to the recovery of gasification products from subterranean coal deposits.
Details of various coal gasification and liquefaction processes may be found in the Encyclopaedia of Chemical Technology, Kirk-Othmer, 3rd Edition (1980) Volume 11, pages 410-422 and 449-473.
In relation to oil (petroleum) recovery, depleted wells or low natural drive reservoirs may be worked by the process of secondary recovery which involves enhancing or inducing a drive in the reservoir by water flooding or in situ combustion. The latter process in elementary form involves lowering an igniter into a bore hole and triggering an ignition of the hydrocarbons in the target reservoir. Although lighter hydrocarbons are consumed in the combustion, the resulting thermal front lowers the viscosity of the heavier deposits and drives them through the formation to a recovery well. Other methods, the so-called tertiary recovery methods, including steam injection, air injection, displacement by polymer introduction, explosive fracturing, hydraulic fracturing, carbon dioxide injection, chemical processes including introduction of caustics have all been proposed for use.
Currently, the industry has available secondary recovery methods that can be classified broadly as xe2x80x9cGas injectionxe2x80x9d, xe2x80x9cWater Floodingxe2x80x9d, and xe2x80x9cThermal Recoveryxe2x80x9d.
xe2x80x9cGas Injectionxe2x80x9d techniques inject a gas, such as nitrogen or carbon dioxide, into the target formation to elevate pressure upon the residual oil and facilitate production thereof.
xe2x80x9cThermal Recoveryxe2x80x9d techniques require injection of an air/oxygen mixture into the formation toward a heating element at the base of the string. Whenever the critical conditions of air/oil and heat are reached the oil ignites and produces a combustion front. The front is driven in the desired direction by continuing the supply of combustion-supporting gas at a controlled pressure to avoid burn-back. As the combustion front progresses through the oil reservoir, oil and formation water are vaporised, driven forward in the gaseous phase and re-condensed in the cooler section of the formation, in turn the condensed fluids displace oil into the production well bores.
xe2x80x9cGasificationxe2x80x9d processes of the known types can be distinguished by the end product to be recovered. One approach to gasification, subjects the ageing field to a method of gasification of the residual oil so that the resulting gas can be collected, i.e. the gas rather than the residual oil becomes the target product. Another approach relies on the gas produced in the gasification process to act as a fuel in a combustion process (c.f. discussion on thermal recovery above) to displace residual oil to allow it to be retrieved from the formation, i.e. the gas is only a means to enhance recovery of the oil which remains the target product. The latter is a true enhanced oil recovery method (EOR) whereas the former is a gas-producing process (GPP) wherein the oil is volatilised and thermally cracked to gases which are captured and transported to the surface for processing.
In order for the GPP process to be successful, the produced gas must be captured readily, and fields where highly porous formations are situated above the oil would be considered unsuitable for this approach.
An EOR process is only effective if the residual oil deposits are not so heavy as to make flow difficult, and do not contain significant levels of high molecular weight paraffins and waxes which would inhibit flow. Furthermore, the known thermal recovery processes may not perform satisfactorily due to a declining temperature gradient around the igniter which can lead to heavy fractions in the oil consolidating at a distance from the igniter and thus clogging the formation to prevent effective recovery.
An object of the present invention is to provide improvements in or relating to the recovery of oil from partially depleted or ageing xe2x80x9cweak drivexe2x80x9d fields and formations where gasification of residual oil is a potential solution.
A further object of the present invention is to provide an apparatus for in situ gasification of oil to produce a synthetic gas xe2x80x9csyngasxe2x80x9d within the reservoir. An aim in developing such an apparatus is to provide a tool adapted to be readily launched into the reservoir using existing well access or requiring minimal adaptations thereof.
A still further object of the present invention is to provide a method of secondary recovery or enhanced oil recovery offering advantages over prior art proposals.
Yet another object of the present invention is to provide according to one aspect a gasification process to be performed on the production platform.
Further objectives of the present invention include the provision of methods of gas production and oil recovery, which obviate or mitigate problems evident or inherent in known methods.
Thus according to the present invention there is provided a process for in situ gasification of mineral oil in a subterranean formation which comprises running a tool having a controllable thermal device therein from a surface production facility down to the subterranean formation, bringing said tool into operational proximity with the mineral oil in said subterranean formation, and activating the tool to operate the thermal device within a predetermined temperature range to generate gases or oily vapours from said mineral oil.
According to one aspect the gas and vapours so generated by the thermal gasification process are collected by providing a gas riser tubing between the production facility and the subterranean formation such that an end of said tubing enters the accumulating gas/vapour head space above the oil to provide for gas recovery to the surface production facility.
According to another aspect the gas and vapours so generated by the thermal gasification process are allowed to accumulate above the mineral oil to build pressure, and the mineral oil is collected by providing a production riser tubing between the surface production facility and the subterranean formation such that an end of said tubing penetrates the oil to a sufficient depth to permit oil recovery to the surface production facility.
The latter thermal gasification process is suitable for use in recovery of oil when the formation beneath the oil is substantially impermeable to oil, and the formation above the oil is not significantly permeable to gas generated. Those skilled in the art will recognise that if the formation beneath the oil is permeable to oil to any significant extent oil may be driven further into the permeable formation, and that if the xe2x80x9coverheadxe2x80x9d formation is porous gas generated will simply leak away into the formation. Therefore, those skilled in the art will normally survey and assess the formation and thereafter exercise judgement as to which process according to the present invention is suited to the formation surveyed for oil recovery purposes, or whether an alternative approach needs to be considered. Other factors that those of appropriate experience and skill in this field will take account of is the quality of the oil to be recovered. Heavy crude oil containing high molecular weight paraffins and waxes at significant levels may not be suitable for the purposes of this invention.
Further according to the invention there is provided an apparatus for recovery of oil and/or gas by a process involving an in situ gasification of mineral oil in a subterranean formation which comprises a tool having a controllable thermal device, controllable means for launching (and optionally subsequently recovering the tool) from a surface production facility down to the subterranean formation, logging means for determining the location of the tool in relation to its operational proximity to the mineral oil in said subterranean formation, and at least one riser tubing for the selective recovery of mineral oil, or gaseous or vaporised products from said mineral oil.
The invention further provides a tool for use in gasification of mineral oil in situ in a subterranean formation, said tool comprising a thermal device selected from a spark igniter, an electrically heated coil, an electromagnetic heating device, a pyrotechnic charge with corresponding ignition device, an electrode arc ignition device, and a resistive heating element.